understanding entropy (was: building a true RNG)

[John S. Denker]

> At 10:59 PM 7/22/02 -0700, jamesd at echeque.com wrote:
> 
> >Entropy is not quite a physical quantity -- rather it is on the
> >slippery edge between being a physical thing and a philosophical
> >thing. If you are not careful, you will slip into a deep epistemic
> >bog and find yourself needing to ask "how do we know what is
> >knowable, and what is the whichness of why?"
> >
> >To avoid such deep waters, know where your entropy is coming from.

Right.

Then David Honig wrote:
> 
> We agree on your substantive points re RNGs, I think, 

I join in the agreement.

> but you're interestingly wrong here.  

I don't think jamesd's point was wrong.  One could quibble
about some of the wording, especially if it were taken out
of context, but the passage as a whole makes an important,
valid point.

> Entropy is a physical quantity, it even figures into chemistry.  

Yes, it is a physical quantity.  Yes, it enters into chemistry.
But it also contains an element of subjectivity.

For a careful discussion of what entropy is, including
the element of subjectivity, see
  http://www.monmouth.com/~jsd/physics/thermo-laws.htm#sec-second-law

> The physics-of-computation people (Bennett? Landaur? etc) 

Charles H. Bennett and Rolf Landauer.

> have written about thermodynamics & information.

Not to mention Leo Szilard, Ed Fredkin, Wojciech Zurek,
and others.

> Modulo Chaitin-type mindgames about measuring it :-)

Chaitin's work is profound and well-regarded.  Referring
to it as "mindgames" is, well, nothing but name-calling
and won't advance the scientific discussion.  If anybody
has a thoughtful objection to Chaitin's work I would be
extremely interested to hear it.

> Anyway we're cryptographers, not philosophers, so we should 
> be safe..

A lack of understanding of what entropy is has gotten more
than one cryptographer into trouble.

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